1. Field of the Invention
The present invention relates to a substrate for inkjet printing and a method of manufacturing the same.
2. Discussion of the Related Art
Inkjet printing is one of most significant structuring processes used to produce full-color displays that use light-emitting, semi-conducting polymers (LEPs). The process entails depositing small drops of a solution of the corresponding polymer onto a suitable substrate. The inkjet printing process, however, may also be used to deposit a color filter or a DNA sensor onto a substrate.
These applications require exact placement of a target substance, such as ink, on an activated surface of the substrate. The inkjet printing technique fits this requirement. The ink is prepared by dissolving an active substance in an auxiliary substance, and then depositing the ink onto the substrate as small droplets using a piezo or “bubble jet” inkjet technique. Exact positioning of ink droplets onto the substrate may be achieved by several techniques, including accurately positioning the inkjet head relative to the substrate. A film of the active substance is formed on the substrate as the auxiliary substance evaporates.
A common failure occurring in the printing process is the run-out of ink droplets of the active substance toward neighboring positions on the surface of the substrate. In a display using organic light-emitting diodes (OLED), in which red, green, and blue emitting areas are closely arranged, such a run-out of ink droplets mixes the three colors of the light emitting materials in the emitting areas.
OLED display devices have been known for over 20 years. They are classified into large molecular weight polymer-based OLEDs (PLEDs) and low molecular weight OLEDs (SM-OLEDs). WO00/76008A1 (CDT) describes a basic structure of a PLED display device. U.S. Pat. Nos. 4,539,507 and 4,885,211 (Eastman-Kodak) describe the principle structure of an SM-OLED in which ALQ3 (tris-(5-chloro-8-hydroxy-quinolinato)-aluminum) is used as a light-emitting and electron transport material.
OLED display devices are electroluminescent display devices. In such a device, electrons and holes are injected into a semi-conducting material layer through appropriate contacts, and light is generated by the recombination of the charge carriers.
A piezo inkjet printing technique is typically used to manufacture full-color displays based on polymer OLEDs. In this case, small drops of a solution containing an active substance (hole transfer or light-emitting materials) are deposited on the active surface of a suitable substrate. The active surface (single picture point) for high-resolution displays recently used in mobile phones has dimensions of approximately 40 μm×180 μm.
Conventional inkjet heads produce ink droplets having a diameter of 30 μm. Consequently, ink droplet diameter lies within the dimension of the picture point. In order to prevent an overflow of the ink droplets, the substrate surface may be formed using one of the following methods.
In a first method, a substrate surface is produced so that areas have different surface tensions (energies) providing different covering characteristics to ink applied thereto. A second method uses a geometrical (mechanical) barrier for preventing ink droplet overflow.
One approach is disclosed in EP 0989778 A1 (Seiko-Epson). A material that may induce different surface tensions over the substrate surface is utilized. Printed ink may run in areas with high surface energies because areas with low surface energies act as barriers. OLEDs with a higher surface energy peripheral zone are normally designed in order to obtain a uniform thickness film. The film is homogeneous to the peripheral zone, but it may be much thinner near an outer region of the active zone toward the barrier. Required differences in surface energy can be achieved in many ways.
EP 0989778 A1 (Seiko Epson) describes a two-layered surface structure in which an upper layer has a smaller surface tension, and a lower layer has a larger surface tension. The varying surface tensions may be achieved by a surface treatment using plasma. The lower layer is typically made of inorganic materials such as silicon oxide/nitride.
In this case, the inorganic layer corresponds to the peripheral zone with a larger surface tension, thereby making it easier to deposit a homogeneous polymer film in an inkjet printing process.
Such layers may be deposited and patterned using general semiconductor manufacturing processes, such as sputtering, plasma enhanced chemical vapor deposition (PECVD), etc. These vaporizing processes require long pulse duration and are costly, which offsets a cost advantage gained by using the OLED technology. The second layer has a specific topology including, for example, separators protruding from the surface of the substrate by a predetermined height and having lower surface tensions. Therefore, a polymeric film deposited on the second layer may have an undesirable increase in thickness from the separators toward the peripheral zone, which may reach pixels.
Another disadvantage of EP 0989778 lies with an ink reservoir used for overflow protection. It is time-consuming and more complicated to construct an ink reservoir because additional processes are involved.
JP09203803 discloses a chemical treatment of a substrate surface that is previously treated with photoresist. The photoresist is exposed through a mask and developed. In this structure, the photoresist is applied to an area having a smaller surface tension than the area to which no photoresist is applied. The photoresist structure has an average surface tension on an edge region so that there is no abrupt change in the surface tension of the substrate. However, the surface tension and geometry in this edge region may not be freely, selectively varied, and it has a low spatial dissolution capacity. Also, only one particular photoresist may be used. Surface tension variations, therefore, may not be induced using other materials, which limits applicability. Finally, the chemical treatment considerably increases the overall manufacturing time.
JP09230129 discloses a two-stage surface treatment method including providing the entire substrate surface with a low surface tension and selectively exposing a region of the surface to a short-wavelength of light to increase the exposed region's surface tension. However, this method may lead to limited surface tension variations, and the time-consuming exposure process may not be suitable for mass production.
As described above, geometrical (mechanical) barriers may be formed to prevent an ink droplet overflow. U.S. Pat. No. 6,388,377 B1 discloses a photoresist stripe structure positioned between two neighboring picture points. Each photoresist stripe is more than 2 μm high and serves as a physical barrier to prevent ink droplet overflow. The production of such photo-resist structures is described in EP 0996314 A1. Two photoresist structures (called “banks”), arranged in parallel to one another, form a channel, and picture points emitting red, green, or blue light are interposed therebetween. A picture points layer is formed by printing a suitable ink in the channel, and the photoresist structures prevent ink droplet overflow toward picture points, which lie on outer sides of the channel. The banks have a height that is larger than 0.5×(the width of a picture point/the diameter of an ink droplet), which is larger than the thickness of an active material film deposited using an inkjet printing technique. Finely structured banks may be manufactured by forming round, oval or triangular notch indentations that serve as overflow preventing reservoirs. However, banks at this height may negatively impact a subsequent metal deposition process. The cathode of an OLED structural element may be formed of metal using thermal evaporation or sputtering. A discontinuous metal film or a metal film that is thinner on the side walls of the “banks” may be deposited due to the shape and height of the photoresist structure, thereby increasing electrical resistance, which requires more input power.